Add a null check for cache listener message data

[philipwalton]

R: @jeffposnick

While testing out the latest Workbox RC I noticed that if you run Router#addCacheListener() and then postMessage() to the SW anything that's not an object, it'll fail.

This PR adds an extra check and test that should ensure non-CACHE_URLS messages don't cause errors.

[workbox-pr-bot]

PR-Bot Size Plugin

Changed File Sizes

File	Before	After	Change	GZipped
packages/workbox-routing/build/workbox-routing.prod.js	3.38 KB	3.39 KB	+0%	1.48 KB

New Files

No new files have been added.

All File Sizes

View Table

File	Before	After	Change	GZipped
packages/workbox-background-sync/build/workbox-background-sync.prod.js	3.60 KB	3.60 KB	0%	1.56 KB
packages/workbox-broadcast-update/build/workbox-broadcast-update.prod.js	1.85 KB	1.85 KB	0%	934 B
packages/workbox-build/build/_types.js	41 B	41 B	0%	61 B
packages/workbox-build/build/index.js	3.64 KB	3.64 KB	0%	1.36 KB
packages/workbox-cacheable-response/build/workbox-cacheable-response.prod.js	584 B	584 B	0%	348 B
packages/workbox-cli/build/app.js	5.58 KB	5.58 KB	0%	1.98 KB
packages/workbox-cli/build/bin.js	1.16 KB	1.16 KB	0%	580 B
packages/workbox-core/build/workbox-core.prod.js	5.43 KB	5.43 KB	0%	2.36 KB
packages/workbox-expiration/build/workbox-expiration.prod.js	2.84 KB	2.84 KB	0%	1.24 KB
packages/workbox-google-analytics/build/workbox-offline-ga.prod.js	1.90 KB	1.90 KB	0%	902 B
packages/workbox-navigation-preload/build/workbox-navigation-preload.prod.js	657 B	657 B	0%	321 B
packages/workbox-precaching/build/workbox-precaching.prod.js	4.21 KB	4.21 KB	0%	1.69 KB
packages/workbox-range-requests/build/workbox-range-requests.prod.js	1.52 KB	1.52 KB	0%	762 B
packages/workbox-routing/build/workbox-routing.prod.js	3.38 KB	3.39 KB	+0%	1.48 KB
packages/workbox-strategies/build/workbox-strategies.prod.js	4.86 KB	4.86 KB	0%	1.19 KB
packages/workbox-streams/build/workbox-streams.prod.js	1.38 KB	1.38 KB	0%	682 B
packages/workbox-sw/build/workbox-sw.js	1.34 KB	1.34 KB	0%	745 B
packages/workbox-webpack-plugin/build/generate-sw.js	5.29 KB	5.29 KB	0%	1.84 KB
packages/workbox-webpack-plugin/build/index.js	349 B	349 B	0%	255 B
packages/workbox-webpack-plugin/build/inject-manifest.js	7.22 KB	7.22 KB	0%	2.48 KB
packages/workbox-window/build/workbox-window.dev.umd.js	30.38 KB	30.38 KB	0%	8.01 KB
packages/workbox-window/build/workbox-window.prod.umd.js	4.66 KB	4.66 KB	0%	1.81 KB

Workbox Aggregate Size Plugin

8.83KB gzip'ed (59% of limit)
22.13KB uncompressed

[coveralls]

Coverage increased (+0.03%) to 82.837% when pulling e4c88b1 on cache-listener-fixes into 2d8ce9a on master.